Ion source



Aug. 23, 1955 J. JONES ET AL ION SOURCE 2 Sheets-Sheet 1 Filed Sept. 8, 1950 WWW N t Y 5: er E c N x T PP fin A Aug. 23, 1955 R. J. JONES ET AL ION SOURCE 2 Sheets-Sheet 2 Filed Sept. 8, 1950 ATTORNEY pendent of the translational velocity.

2,715,197 Patented Aug. 23, 1955 ION SOURCE Royce J. Jones and Robert E. Wright, Oak Ridge, Tenn,

assignors to the United States of America as representedby the United States Atomic Energy Commission Application September 8,1950, Serial No. 183,854

3 Claims. (Cl. 313-63) The present invention relates to gaseous electric dis- -charge apparatus, and especially to an improved source of ions for a particle accelerator such as a cyclotron.

The cyclotron is an instrument for accelerating particles to high velocities by combined electric and magnetic fields, and is based upon the principle that the angular velocity of a particle moving in such fields is indelt comprises two .metal Ds, which are semicylindrical chambers shaped like the letter D, insulated from each other in a vacuum chamber disposed between the poles of a magnet so that the magnetic field is perpendicular to the faces of the Ds. Charged particles leave a source near the center :of the Ds and are accelerated into one of the Ds by the action of a high-frequency electric field impressed upon the Ds by an oscillator. Inside the Ds the particles will travel in a circular path at a constant velocity under the influence of the magnetic field. But when the particles reach the gap, they are accelerated thereacross by the electric field, the oscillator frequency and magnetic field strength being so adjusted that the particles require one-half the oscillator period to travel around one D. At each crossing of the gap, the particles are further accelerated, and travel in a larger orbit, until they are led away from the periphery of the B to a target.

.Ions for acceleration can be produced by a stream of electrons between the Ds from a hot filament to an anode, and also by an electric discharge passing through a gas .in a closed chamber. In the discharge type ion source, a capillary passage aligned with the entrance to .the Ds is normally provided, and the. gas is ionized during its passage therethrough. However, neither of the above source types has proven entirely satisfactory for producing intense proton beams, because of beam divergence, low ion currents, and the low ratio of overall protons to other positive ions in the beam. Since the number of the desired type of particles striking the tar- ,get of a particle accelerator can be no greater than the .number of that type of ion produced in and ejected from the source, it is apparent that the overall efiiciency of a source in converting gas molecules to the desired ions will determine to a great degree the elficiency of the accelerator.

It is, therefore, the primary object of our invention to provide an ion source characterized by its unusually high overall efiiciency of ion production.

Another object of our invention is to provide a means for producing a relatively intense current of protons.

A further object is to provide an improved cyclotron source of the hot cathode arc type.

The above objects are attained in the present invention byproviding an intense are inside an elongated'and enclosed chamber, through which are all molecules of the gas to be ionized must pass before reaching the arc chamber. The gas is admitted behind the filament, and flows around the filament, and through the are into the chamber. Electrodes are provided outside a long, .narrow slit in the chamber to draw ions out of the chamber and to focus and define the beam before it enters the cyclotron D.

A preferred form of apparatus with which the above objects may be accomplished is illustrated in detail in .the appended drawings, in which:

Fig. 1 illustrates schematically our improved source in place between the Ds of a cyclotron,

Fig. 2 is a sectional view illustrating the construction of .a preferred embodiment of our improved ion source adapted for use in a cyclotron.

Fig. 3 is an enlarged sectional view illustrating the anode assembly of our improved ion source.

Fig. 4 is an isometric view, partly in section, illustrating the assembly of the filament and cooling tubes, and

means for holding said assembly in position,

Fig. 5 is a sectional view taken along line 55 of Fig. 4,

Fig. 6 is an isometric view illustrating the accelerating and focusing electrode 7 shown in Fig. 2.

Referring now to Figs. 2 and 3, the source includes an elongated tubular envelope 1 forming the arc chamber. The anode assembly includes a disc 2, mounted on .stem 20, around which an insulating sleeve 21 fits snugly. The .sleeve is press fitted inside the envelope 1, so that the entire assembly forms a substantially gas-tight joint, closing one end of the envelope 1. A bulbous, bellshaped housing 3 carrying flange 11 thereon extends from the opposite end of the envelope 1 and defines a filament chamber. As may be seen from Figs. 4 and 5, one leg of an electron emissive filament 5 is clamped in a slot .54) in a filament supporting member 6, while the opposite leg is clamped in a similar slot .50 in the support .6. The .filament extends into the filament chamber in direct alignment with both the anode 2 and a slot or aperture 4. A U-shaped tantalum filament .170 inch in diameter is preferred, but'it is apparent that other thermionic filaments, such as Wolfram, of greater or lesser diameter, and shaped in various configurations could also be employed. The relatively large cross-section of the filament prevents the magnetic forces from warping .it and thus eliminates the need for radio-frequency heating.

The collimating slot 4, for optimum results, should be located so as .to be tangent to an imaginary line along the inner edges of the arc chamber aperture slit 13 as shown inFig. 2. The collimating .slot is desirably .142

inch in diameter and X inch in depth. These dimensions, which are critical for maximum source efiiciency, are very close to optimum since the use of a larger and .shallower hole decreases the proton output efficiency, while .a smaller, deeper hole does not allow sufficient positive ions to reach the filament 5 to maintain a stable arc.

Avsubstantially gas-tight metal box 8 forms a chamber into which the charge gas, such as hydrogen, may be introduced through entry port 9. Annular ring 10, threaded around its periphery and recessed to receive flange 11,

engages a corresponding threaded cylindrical recess 12 in the wall 14 of the box-8 .to-hold the chambers firmly in place.

Supports6 and-6, having .blindholes .36, 31 and counterbores 32,33 arerigidl-y fastened to wall 14 .of box 8 by means of tapered insulator wedges .36, 37 .and screws .38 and 39. Tubes 26, 27 are brazed into .counterbores 32, 33. Within the tubes 26, 27 and coaxial therewith extend coppertubes 34, 35, of a substantially smaller diameter, which terminate at apoint near the filament clamps. Cooling water may enter through tubes 34, 35 and be expelled through holes 30, 31 and .tubes 26, 27. Electrica'l leads from a source of filament power may be connected to the inner tubes 34, 35, which should be made of copper or other good electrical conductor.

Sheet 52 of mica or other suitable insulator is disposed between the wall 14 and the supports 6, 6', and sheet 53 of like material is disposed between the supports 6, 6' to prevent shorting of the filament supply, through wall 14 to the housing 3 or through the conductive supports, respectively. The insulator wedges 36, 37 may be Melamine, Teflon, or other suitable electrical insulator possessing good structural strength, and are provided to insulate the filament structure from wall 14 and screws 38, 39, which assume the potential of housing 3, and therefore differ in potential from the filament by the electron accelerating voltage used to strike and maintain the arc. The sleeve 21 is preferably quartz to insulate envelope 1 from item 20.

The envelope 1 has an ion exit slit or aperture 2 /2 inches long disposed along its surface. Adjacent the slit, thewall is cut away to a thickness of .018 inch.

An electrode 7, shown in Fig. 6, is rigidly mounted on the lower D 17 and disposed in confronting relation with the slit 13. The slit 54, inch wide, is thereby maintained substantially /3 inch from the lip of the exit slit 13.

The electrode 7, the tubular envelope 1, housing 3 and ring 10 may preferably be carbon, but other conductive materials which will withstand high temperatures,

such as stainless steel, might also be employed. Carbon,

however, will be less affected by the arc and ion beam.

Operation of our invention may best be understood from the following description, when read in conjunction with the appended figures.

The above listed dimensions are those of a preferred embodiment of our invention, and are those which lead to optimum results with the apparatus illustrated. They should not be considered as critical in the absolute sense;

through aperture '4 into the chamber 1 bythe potential between the filament and grounded envelope by the copious quantity of electrons emitted by the filament, and the partial neutralization of the space charge around the filament by the positive ions resulting from electron collisions with the gas molecules as the electrons oscillate through the are. This are is collimated by a large magnetic field directed in the same direction as the long dimension of envelope 1; that is, into the paper in Fig. 1. This field may be, for example, 6,0008,000 oersteds, and may be provided by the large electromagnets of the cyclotron, shown schematically in Fig. 2.

Ions are removed from the arc chamber through slit 13 and accelerated'toward lower D 17 by an electrostatic field set up by accelerating and focusing electrode 7, which 7 is maintained at the lower D potential with respect to envelope 1 by a suitable power source or oscillator 15. After entering the lower D, the particles are accelerated by-the'cyclotron in the conventional manner.

In considering the usefulness of an ion source, the rate at which the source produces ions is a prime consideration. It is apparent that if a higher pressure could be maintained in the neighborhood of the filament, more molecules would be subject to ionization, and a greater number of ions would result. Counterbalancing that effect, however, is the greatly increased scattering of the ion beam resulting from the higher pressure outside the are, so that the useful beam intensity is limited to an undesirably small magnitude. We have overcome the seeming impasse due to the above conflicting pressure effects byproviding an elongated tubular arc chamber, closed at 4 one end by an anode and communicating at the opposite end with the filament chamber, and communicating with the evacuated space between the cyclotron Ds only by a long, narrow slit. By means of our arrangement, we may maintain a relatively high gas pressure in the arc,

and so get maximum ion production; yet we also may keep a relatively low gas pressure outside our arc chamber so that we avoid excessive scattering of our ion beam.

The quality of ion beam which leaves the exit slit in the are chamber may be further improved by the provision of two accelerating electrodes outside the slit. The electric field they establish in effect draws positive ions out of the arc chamber and accelerates them toward one D. An essential function of the electrodes is to define and focus the beam, greatly increasing the percentage of ions produced which may be effectively used to bombard a target, and minimizing beam scattering at the ion exit slit due to the pressure outside the chamber. In open arcs, the arc plasma will tend to bow away from the electric accelerating field, causing the beam to diverge from the source. But in our improved source, the electric field established by the accelerating electrodes reaches past the thin lips of the ion exit slot to maintain the plasma more nearly parallel to the slot, effectively reduc ing such divergence.

The gas feed system of our invention provides that the gas enter behind the filament, flow around it into a critically-positioned collimating slot, then enter the arc chamber. By introducing the gas at the filament, an arc may be struck across the chamber with only a relatively small gas flow rate, such as l-1O c. e. per minute. If the 'gas were fed directly into the arc chamber, for example, not enough positive ions would reach the filament to maintain the are, even though it would strike, at desired gas flow rates. In addition, mounting of the source unit between the Us is simplified when the gas enters at the filament. The collimating slot referred to above serves to define the arc, and, in addition, provides the sole exit port for electrons from the filament and for gas molecules to be ionized. through the are before reaching the arc chamber, and

greatly increases the probability that a given molecule will be ionized.

While a preferred embodiment of our improved ion source has been described in connection with a cyclotron,

it will be apparent to one skilled in the art that various combination of the novel features herein illustrated may be adapted for use in other particle accelerators and the like. Therefore, our invention is not to be construed as limited to the embodiment herein described, but only by the scope of the appended claims.

We claim:

1. In a cyclotron, a pair of confronting accelerating D electrodes, means for establishing a magnetic field normal to the confronting faces of said electrodes, an ion source comprising a longitudinally apertured elongated hollow cylinder defining an arc chamber disposed parallel to the direction of the magnetic field and equidistant from the accelerating electrodes of said cyclotron, the

longitudinal aperture of said chamber providing an exit through the arc chamber, a gas chamber communicating v with said filament chamber and having a port for receiving 7 gas from a source, and an electrode member disposed parallel to said are chamber and in spaced relation to the ion It thus insures that every molecule must pass.

exit slit of said chamber and defining a second slit for passage of ions emitted through said ion exit slit.

2. An ion source of the character described comprising a tubular envelope forming an arc chamber; a first housing of substantially greater diameter than said envelope connected to one end thereof and forming a filament chamber, one wall of said housing being provided with a first passageway of smaller diameter than said envelope and communicating with both of said chambers; a second housing forming a gas chamber and adapted to receive one end of said first housing in substantially gastight relation; a support member; and a filament mounted on said support member within said second housing and extending into said first housing in longitudinal alignment with said passageway; said support member comprising first and second blocks of electrical insulating material provided with at least one inclined side and mounted on said second housing with said sides in spaced apart, confronting relation; third and fourth blocks of electrically conductive material provided with correspondingly inclined sides disposed between and abutting against said confronting inclined sides, with blind passageways therein for receiving a coolant, and with mounting slots for said filament, and being adapted to conduct electric current to and from said filament; and respective sheets of electrical insulating material disposed between adjacent sides of said conductive blocks and between said blocks and said second housing.

3. An ion source of the character described comprising a tubular envelope forming an arc chamber; a first housing of substantially greater diameter than said envelope connected to one end thereof and forming a filament chamber, one wall of said housing being provided with a first passageway of smaller diameter than said envelope and communicating with both of said chambers; a second housing forming a gas chamber and adapted to receive one end of said first housing in substantially gas-tight relation; a support member; and a filament mounted on said support member within said second housing and extending into said first housing in longitudinal alignment with said passageway; said support member comprising first and second wedges of electrical insulating material secured to said second housing in confronting relation; third and fourth wedges of electrically conductive material, correspondingly tapered to conform to the slopes of said first and second wedges, abutting thereagainst and provided with apertures for receiving opposite extremities of said filament and with blind passages for receiving a coolant; respective sheets of electrical insulating material disposed between said third and fourth wedges and between said wedges and said second housing; and means for delivering both electrical power and coolant to said third and fourth wedges comprising first and second electrically conductive coaxial tubes, the outer tubes thereof being connected to corresponding conductive wedges at the entrances to said passages and the inner tubes thereof extending into corresponding passages for delivery of coolant.

References Cited in the file of this patent UNITED STATES PATENTS 1,948,384 Lawrence Feb. 20, 1934 2,427,484 West Sept. 16, 1947 2,498,841 King Feb. 28, 1950 OTHER REFERENCES Physical Review, volume 48, December 1, 1935, pages 886-892, High Current Ion Source for Nuclear Investigations by Lamar et al.

Physical Review, volume 51, June 1, 1937, pages 936-941 Low Voltage Proton Sources by Lamar et al.

Review of Scientific Instruments, volume 10, February, 1939, pages 63-67, A Capillary Ion Source for the Cyclotron by Livingston et al.

Physical Review, volume 62, July 1 and 15, 1942, pages 11-18, Proton Production by Electron Collisions in Molecular Hydrogen by Newhall. 

1. IN A CYCLOTRON, A PAIR OF CONFRONTING ACCELERATING D ELECTRODES, MEANS FOR ESTABLISHING A MAGNETIC FIELD NORMAL TO THE CONFRONTING FACES OF SAID ELECTRODES, AN ION SOURCE COMPRISING A LONGITUDINALLY APERTURED ELONGATED HOLLOW CYLINDER DEFINING AN ARC CHAMBER DISPOSED PARALLEL TO THE DIRECTION OF THE MAGNETIC FIELD AND EQUIDISTANT FROM THE ACCELERATING ELECTROCES OF SAID CYCLOTRON, THE LONGITUDINAL APERTURE OF SAID CHAMBER PROVIDING AN EXIT SLIT FOR IONS FORMED THEREIN, A BELL-SHAPED EXTENSION OF ONE END OF SAID CYLINDER DEFINING A FILAMENT CHAMBER, ANODE AND FILAMENT ELECTRODES FOR PRODUCING AN ELECTRIC ARC THROUGH SAID CHAMBERS, SAID FILAMENT BEING DISPOSED WITHIN SAID FILAMENT CHAMBER AND SAID ANODE BEING DISPOSED AT THE DISTAL END OF SAID ARC CHAMBER AND ELECTRICALLY INSULTED THEREFROM, MEANS DEFINING AN APERTURE BETWEEN SAID FILAMENT CHAMBER AND SAID ARC CHAMBER, MEANS FOR ESTABLISHING AN ARC DISCHARGE IN SAID CYLINDER FROM SAID FILAMENT, SAID APERTURE BEING ALIGNED WITH AND IN SPACED RELATION TO SAID FILAMENT FOR COLLIMATING THE ARC THROUGH THE ARC CHAMBER, A GAS CHAMBER COMMUNICATING WITH SAID FILAMENT CHAMBER AND HAVING A PORT FOR RECEIVING GAS FROM A SOURCE, AND AN ELECTRODE MEMBER DISPOSED PARALLEL TO SAID ARC CHAMBER AND IN SPACED RELATION TO THE ION EXIT SLIT OF SAID CHAMBER AND DEFINING A SECOND SLIT FOR PASSAGE OF IONS EMITTED THROUGH SAID ION EXIT SLIT. 